Spectroscopy, which broadly refers to the interaction between energy and matter, may be used for such purposes as chemical and biological sensing. In a typical spectroscopy measurement, incident radiation (photons, for example) is directed to a particular analyte (i.e., a species, molecule, compound, biological or non-biological specimens, or, in general, matter being analyzed).
Raman spectroscopy refers to the study of vibrational, rotational and other specific modes of an analyte when incident photons scatter inelastically as a result of their interaction with the analyte. The scattered photons have a frequency that may be relatively low (called Stokes scattering), or relatively high (called anti-Stokes scattering), as compared to the frequency of the incident photons. The absorption of the incident photons and the resulting shifts in the wavelengths of the inelastically scattered photons as well as the relative peaks in the Raman emission are unique characteristics of the analyte. Hence, Raman spectroscopy has wide application for chemical and biological sensing, in particularly, in a portable system.
An analyte can also emit photons through various luminescence processes. In the fluorescence process (a relatively short-lived luminescence process, as compared to the phosphorescence process, which is luminescence from relatively long-lived states), the analyte emits photons as a result of absorbing incident photons by the molecules of the analyte. Similar to the Raman process, the wavelength of the re-emitted photons can be different from that of the incident photons. Stokes fluorescence is the re-emission of longer wavelength photons (lower frequency or energy). This energy difference is the Stokes shift. In the fluorescence process, an analyte can reemit photons with higher energy than the incident photon, and the energy difference is called an anti-Stokes shift (also called “up-conversion”). In the fluorescence and the other types of luminescence emission processes (e.g., bioluminescence and chemiluminescence processes), the absorption and the re-radiation (re-emission) of the photons together with the wavelength distribution of reemitted photons are unique characteristics of a particular molecular structure of the analyte.